In recent years, there are increasing demands for lenses for constituting image-sensing optical systems of digital cameras, and the like. Under the circumstances, processes called precision press-molding or mold optics shaping are attracting attention as a lens production process capable of supplying a large amount of lenses formed of a glass in a small delivery period. However, an optical glass for precision press-molding is required to have not only properties required of general optical glasses but also the property of being softened at a low temperature (“low-temperature softening property” hereinafter), and such optical glasses constitutes a unique region clearly distinguishable from the region of conventional optical glasses.
On the other hand, from the viewpoint of optical designing in an image-sensing optical system, there is demanded a lens formed of a high-refractivity and high-dispersion glass, and there are increasing demands for glass materials for providing such lenses by the above precision press-molding.
For example, such a glass is disclosed in JP-A-2003-160355.
The invention disclosed in the above JP-A-2003-160355 provides a glass containing components such as P2O5, Nb2O5, Bi2O3, TiO2, WO3 and an alkali metal oxide, so that there is materialized a high-refractivity and high-dispersion glass having the low-temperature softening property that the precision press-molding requires.
In the precision press-molding, there is first prepared a preliminary shaped material that is formed of an optical glass having a weight precisely in agreement with the weight of a precision press-molded product as an end product and that is called a preform having a form (e.g., a sphere) suitable for press-molding, and the preliminary shaped material is heated and pressed with a press mold.
In the invention disclosed in the above JP-A-2003-160355, a molten glass is cast into a mold (die), glass gobs having a predetermined weight each are cut from the obtained glass, and preforms are prepared therefrom by a polishing method.
However, such a method requires much labor and a time and also increases a production cost, and the problem is that it is difficult to comply with the demand for the above preforms and optical elements. In contrast, when there can be materialized a hot preform shaping method in which a molten glass in a necessary amount is prepared and caused to flow out of a flow pipe, molten glass gobs are separated one after another from the molten glass flowing out of the flow pipe and the molten glass gobs are shaped into preforms before the they are cooled to solidness to produce a number of the preforms one after another, the preform production can be remarkably improved in productivity as compared with the above method. Further, when this hot preform shaping method is combined with a float-shaping method in which the above molten glass gob is shaped into a preform while the glass gob is caused to float by applying air (gas) pressure thereto, there can be produced a preform of which the entire surface is smoother.
The hot preform shaping method and the float-shaping method are excellent as described above. However, a glass for forming preforms is further required to have hot preform shapeability. That is, it is required to satisfy the following conditions. A molten glass is to be caused to flow out of a pipe in a temperature region in which the glass is not devitrified. A glass gob having a predetermined weight is to be separated without using a cutting blade. A glass gob is to be caused to float by applying air (gas) pressure.
When a glass can be imparted with excellent hot preform shapeability, not only the hot shaping and float-shaping of a preform are made possible, but also there can be prevented a decrease in yields which is caused by devitrification, etc., when a molten glass is formed into a glass.